This invention relates to gas filtration media and especially air filtration media. More particularly, this invention relates to a high efficiency non-woven gas filtration media made by combining two or more different layers into a unitary structure in such a way that the resultant media exhibits improved filtration performance and the media can be pleated.
Filtration media has been developed that will remove particles smaller than 10 microns from gas streams, e.g. air. Filters employing such media are commonly referred to as HEPA (High Efficiency Particulate Air), ULPA (Ultra High Efficiency Particulate Air), and ASHRAE (American Society for Heating, Refrigeration and Air Conditioning Engineersxe2x80x94Standard 52.2) filters.
It is often desirable in high efficiency filters to pleat the filter media. The pleated media is placed in a filter frame to form a completed filter. In such case, the filter media surface area is substantially greater than the planar area of the filter frame. For example, U.S. Pat. No. 5,993,501 discloses composite filter media, suitable for pleating, in which a fibrous filtration layer, preferably made of air laid glass fibers, is supported with a non-woven air laid scrim.
Today, the usual standard high efficiency filter media is that of glass fibers with binders, optionally combined with cellulosic fibers. The media is made by a wet laid process. More specifically, high efficiency filtration media has been made of borosilicate glass microfibers bound together by acrylic resin and produced by a wet laid papermaking process. This provides media that exhibits good filtration performance while exhibiting the physical characteristics required for constructing a pleated media pack for assembly into a rigid filter frame.
More recently, developments in synthetic (polypropylene, polyester, etc.) fibers have provided fibers that can be used in combination with or in place of borosilicate glass and cellulose fibers. It is well known that the efficiency of filtration of small particles e.g. 10 microns or less, is greatly increased by the use of, at least in part, electrically charged fibers in the filtration media. For instance, U.S. Pat. No. 5,955,174 describes electrically charged fibrous filtration media. Composites of a charged layer and a layer of carded fibers or spunbonded fibers are thermally bonded, or adhered, or hydroentangled, or needle-punched to the electrically charged layer.
While electrically charged layer, as noted above, produce increased efficiencies for filtering small particles, these filters also have some substantial disadvantages. First, electrically charged filtration media is typically an air laid media, since subsequent charging of the media must be in connection with a very dry and electrically insulating material. Hence, typically, the electrically charged filtration media is a polyolefin meltblown or spunbonded filtration media.
Air laid filtration media by the nature of the process is not structurally very strong. In addition, as explained in more detail below, the air laid media is not sufficiently stiff to adequately pleat. In some cases, pleats will not be retained in the pleated configuration during usual handling and, in other cases, the desired pleated configuration cannot be maintained during use, since the pressure drop of gas passing through the media causes a collapse of the pleats and an inward folding of the pleats against each other. Therefore, typically, the electrically charged, air laid filtration media is attached to a stronger support, which can be a carded web, a scrim, a spun bonded, a spun lace, layers of meltblown, netting, metal mesh and the like. However, if these supports are too thick or too stiff, it is difficult or impossible to adequately form the filter media into a pleated configuration. Usually, from about 3 to 8 pleats per inch(per 2.54 cm) are desired, but up to 10 to 12 pleats can be used. If the media is too thick, that number of pleats can not be obtained without one pleat folding against another pleat. If the media is too stiff, the media can not be so pleated without breaking, cracking or disrupting the media, especially with angular pleats.
In addition, there are several post treatment methods which can be used to adjust the properties of the air laid media. These include, but are not limited to, calandering, thermal bonding, and binders. However, these post treatments increase the cost and can substantially lower the filtration performance.
In addition, since an air laid filtration media is usually relatively thin, it is often necessary to construct the media using multiple layers of the media, e.g. 3-6 layers, so as to provide acceptable filtration efficiencies, especially when the filter is intended to be a HEPA or ULPA or ASHRAE filter. This, in combination with a support makes the filter media dificult to adequately pleat because of its thickness.
Further, the supports of usual prior art media which are suitable for pleating provide little additional filtration and are, therefore, a substantial additional expense and weight without a substantial compensating effect on filtration performance.
It would, therefore, be of substantial advantage to the art to provide a pleatable, electrically charged filtration media which can easily meet HEPA, ULPA and ASHRAE standards but which does not have the disadvantages described above in connection with conventional, pleatable filtration media with conventional supports. It would be a further advantage to the art to provide such filtration media with increased filtration performance as compared with conventional filtration media.
The present invention is based on several primary and subsidiary discoveries.
Firstly, as a primary discovery, it was found that a pleatable, high-efficiency, non-woven filtration media could be prepared by a combination of an electrically charged air laid fibrous layer and a wet laid fibrous layer. This was surprising in that the art had never considered that wet laid fibrous material could be a candidate for a support for pleated air laid media.
Secondly, as a primary discovery, it was found that, in order to make the electrically charged air laid fibrous layer and the wet laid fibrous layer into a combined filter media which could be pleated, the thicknesses of the two layers must be within certain ranges and the thickness of the combined layers must also be within certain ranges.
As a subsidiary discovery, it was found that, with such air laid and wet laid layers of those certain thicknesses, the two layers could be adequately combined by usual techniques, e.g., adhesives, spot-welding and the like, which would, nevertheless, allow the air laid/wet laid composite filtration media to be adequately pleated without delamination of the constituent layers.
As a third primary discovery, it was found that, in order to achieve such pleating of combined layers making up the filtration media, the stiffness of those combined layers must be between about 200 and 3500 mgs. It was found that with a stiffness of less than about 200 mgs, the filter media is simply not stiff enough to be formed into a retainable pleat. At stiffnesses above about 3500 mgs, no substantial improvement in pleatability was achieved and additional stiffness only increased costs.
As a fourth primary discovery, it was found that the combined layers must have a Frazier of between about 10 CFM(0.28 CMM) and about 400 CFM(11.33 CMM). At a Frazier of about below 10, the filter media exhibits too high a pressure drop for adequate performance of the filter and above about 400 CFM, the filter is too open to achieve high efficiency filtration.
As a fifth primary discovery, it was found that within the ranges noted above, it was possible to produce a filter media with an alpha (xcex1) of at least 15.
As a further subsidiary discovery, it was found that the media of the invention provided pleatability as good as the conventional borosilicate glass fiber media, described above, which is the standard in the art, but with substantially improved filtration performance.
It was also found that with the filter media described above, a filter can be constructed of that media where the side of the filter media having the wet laid layer first contacts a gas stream to be filtered. By having the present wet laid media first contact the gas stream, the more coarse (open) and generally thicker wet laid media of the invention intercepts and removes large particle size solids from the gas stream. Further, the present wet laid layer loads in depth i.e., particles go into and attach to the filter rather than attach just on the face thereof. Typically, in such an arrangement, the present wet laid layer collects in depth solids having particle sizes greater than about 10 microns, and the electrically charged air laid layer (which is downstream of the wet laid layer in the gas stream) collects somewhat in depth solids having a particle size less than about 10 microns. By this arrangement, substantial dust holding capacity (DHC) values, e.g., greater than 15 grams per square meter of media area can be achieved even with a pleatable filter.
Accordingly, briefly stated, the present invention provides a pleatable, high-efficiency, non-woven gas filtration media. The media comprises an electrically charged air laid fibrous layer having a thickness of between about 2 mils(0.05 mm) and about 35 mils(0.89 mm); a wet laid fibrous layer having a thickness of between about 5 mils(0.27 mm) and about 35 mils; and wherein the combined layers have (a) a thickness of between about 10 mils(0.25 mm) and about 50 mils(1.27 mm), (b) a stiffness of between about 200 and about 3500 mgs, (c) a Frazier of between about 10 and about 400 CFM, and (d) an xcex1 of at least 15.
The invention also provides media in a pleated form, as well as a filter where the media is in a pleated form.